The Copula is the asserting element in the sentence. It may be (1) a single word (John is running), (2) a group of words (John has been running), or (3) it may be combined with the predicate attribute (John runs).

One word often does two kinds of work in a sentence, as in the last one, and so is both predicate attribute and copula, just as one man often does two kinds of work in a society, and so is both secretary and treasurer. In the sentence, John runs, the word runs represents the action that is asserted, and therefore is the predicate attribute. It also asserts, and is therefore the copula. Notice that if we change runs to running, we do not have a sentence. Running merely expresses the action; it does not assert it.